g
Surface moisture fluxes and moist baroclinic cyclogenesis
Annick Terpstra
Kvalheim, 8 March 2019 – Winter School on the Influence of Diabatic Processes on Atmospheric Development
WRF
Idealized baroclinic channel
- domain: 7500x2000x25 km - hor. resolution: 20x20 km - vert. levels: 61
- periodic zonal BC - parameterization:
- microphysics [Lin]
- cumulus [KF]
- boundary layer [YSU]
Experimental setup:
- symmetric zonal uniform jet - surface temp. ~273 [K]
- tropopause height: ~6.5 [km]
- f-plane: f=1.36e-4 [s-1] ~70N - surface rel. hum.: 80 %
- perturbation: surface based, cyclonic, warm perturbation
x
Idealized baroclinic channel
Experimental setup
Experimental setup:
- symmetric zonal uniform jet - surface temp. ~273 [K]
- tropopause height: ~6.5 [km]
- f-plane: f=1.36e-4 [s-1] ~70N - surface rel. hum.: 80 %
- perturbation: surface based, cyclonic, warm perturbation
Idealized baroclinic channel
Sensitivity experiments
air-sea temperature difference:
→ dT = [0,2,4,6,8] K
→ adjust only SST
warm cold
Cyclogenesis
Time evolution without surface fluxes
Cyclogenesis
Time evolution with surface fluxes dT=6K
Cyclogenesis
Evolution with and without surface fluxes
without surface fluxes with surface fluxes dT=6K
Minimum sea level pressure
Sensitivity experiments: dT=SST-SAT
intensification phase
dT = 0, 2, 4, 6, 8
Cyclone energetics
Diabaticaly generated APE dominates intensification
*10-7 [m s-1 ]
dT = 6 K
diabatic > baroclinic
APEAPE EKEEKE
diabatic diabatic
baroclinic baroclinic
Ge
Ca
Ce
Moisture fluxes
Framework
Note: advective vapor tendencies >> parameterized vapor tendencies
Surface fluxes & moisture content
Boundary layer flux
HL (shading) [W m-2]
HS (white lines) [W m-2] precipitation [mm]
specific humidity at 2m [gr kg-1]
slp’ (lines, hPa) wind 10m (arrows)
Surface fluxes & moisture content
Boundary layer flux
moisture flux at top of boundary layer [kg m-2 s-1]
HL (shading) [W m-2]
HS (white lines) [W m-2] precipitation [mm]
specific humidity at 2m [gr kg-1]
slp’ (lines, hPa) wind 10m (arrows)
Moisture flux out of the boundary layer top
Sensitivity experiments: dT=SST-SAT
dT = 0, 2, 4, 6, 8
intensification phase
[kg s-1]
Surface fluxes & moisture content
Boundary layer flux
HL (shading) [W m-2] HS (white lines) [W m-2]
specific humidity at 2m [gr kg-1]
blue=outward red=inward
radial moisture fluxes at 400m [shading, kg m-2 s-1]
Surface fluxes: moisture flux
Radial fluxes
blue=outward red=inward
N
E
S W
radial moisture fluxes at 400m [kg m-2 s-1]
radial moisture fluxes along outer boundary [kg m-2 s-1]
height [m]
dT = 0, 2, 4, 6, 8
intensification phase
Radial moisture flux into the boundary layer
Sensitivity experiments: dT=SST-SAT
[kg s-1]
Summary
Role of surface moisture flux
Cold sector:
• strong surface HL
• outward moisture flux
Warm sector
• weak surface HL
• inward moisture flux diabatically
dominated intensification increase in SST →
increase moisture fluxes
surface sensible heat flux SST
Sinclair et al. 2010
More realistic mid-latitude cyclones
warm sector:
negative heat fluxes
Quasi-realistic mid-latitude cyclones
Rudeva & Gulev, 2011
cyclones dry-out during their life-cycle
time
composites of N-Atlantic cyclones (reanalysis data)
→ uselessness of local surface moisture fluxes for cyclone-intensification?
P>E; locP≠locE
→ local cyclone-fluxes are not larger than background
surface fluxes
→ cyclones generated over GS are ‘empty’ and die before reaching Europe (anticyclone area pre-conditions for next cyclone)
from Mark from Heini
Effect of surface moisture fluxes
propagation direction
Rudeva & Gulev, 2011:
“SST distribution determines where high surface fluxes occur, atmospheric configuration when these high surface fluxes occur”
1. timing
Effect of surface moisture fluxes is delayed
* moisture needs to be transported upward, before latent heating can fuel the storm
* surface fluxes prior to intensification phase are more important than during intensification
(Kuo and Reed, 1988, Kuo et al. 1991, Reed and Simmons, 1991, Kristjansson and Thorsteinsson 1995, Reed and Albright 1986, Zhang etal 1999, Gyakum and Danielson 2000)
2. location
Contribution from non-local surface moisture fluxes
* sign and magnitude of surface fluxes follows closely the developing cyclone
* cold-sector & anticyclonic area important source of surface moisture input